kernel: Treat aborting by main() as a fatal system error

An application-supplied main() routine is now considered to be
essential to system operation. Thus, if main() experiences an
error that aborts the main thread a fatal system error is raised.

Note: If main() completes its work and does a standard return-
to-caller the main thread terminates normally.

Change-Id: Icc9499f13578299244a856a246ad2a7d34a72f54
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>

doc/kernel_v2/threads/system_threads.rst

@@ -17,9 +17,7 @@ The kernel spawns the following system threads.
 
 **Main thread**
     This thread performs kernel initialization, then calls the application's
-    :cpp:func:`main()` function. If the application does not supply a
+    :cpp:func:`main()` function (if one is defined).
-    :cpp:func:`main()` function, the main thread terminates once initialization
-    is complete.
 
     By default, the main thread uses the highest configured preemptible thread
     priority (i.e. 0). If the kernel is not configured to support preemptible
@@ -27,10 +25,10 @@ The kernel spawns the following system threads.
     priority (i.e. -1).
 
     The main thread is an essential thread while it is performing kernel
-    initialization, which means a fatal system error is raised if the thread
+    initialization or executing the application's :cpp:func:`main()` function;
-    aborts. Once initialization is complete the thread is considered
+    this means a fatal system error is raised if the thread aborts. If
-    non-essential, so the termination or aborting of a :cpp:func:`main()`
+    :cpp:func:`main()` is not defined, or if it executes and then does a normal
-    function supplied by the application does not cause a fatal system error.
+    return, the main thread terminates normally and no error is raised.
 
 **Idle thread**
     This thread executes when there is no other work for the system to do.

kernel/unified/init.c

@@ -200,8 +200,6 @@ static void _main(void *unused1, void *unused2, void *unused3)
 
 	_init_static_threads();
 
-	_main_thread->flags &= ~K_ESSENTIAL;
-
 #ifdef CONFIG_BOOT_TIME_MEASUREMENT
 	/* record timestamp for kernel's _main() function */
 	extern uint64_t __main_tsc;
@@ -219,6 +217,9 @@ static void _main(void *unused1, void *unused2, void *unused3)
 	k_thread_priority_set(_main_thread, MDEF_MAIN_THREAD_PRIORITY);
 #endif
 	main();
+
+	/* Terminate thread normally since it has no more work to do */
+	_main_thread->flags &= ~K_ESSENTIAL;
 }
 
 void __weak main(void)